1. Field of the Invention
This invention pertains in general to semiconductor manufacturing using a wet selective etch process and in particular to a method and apparatus for agitating the etchant used in such a process.
2. Description of Background Art
Selective etching is a process in which selected material is removed from a silicon substrate or from thin films on the substrate surface. In one type of selective etching, a mask layer is used to protect specific regions of a substrate on a wafer surface. Then, a selective etch removes material not covered by the mask. In another type of selective etching, self-aligned silicide (salicide), a metal is deposited over the substrate structure and reacted with exposed silicon areas to form a silicide. Then, a selective etch removes the unreacted metal without attacking the silicide.
In a wet selective etching process, the substrate is etched through exposure to a liquid etchant. The liquid etchant removes a predetermined amount of the uncovered or unreacted material and, thereby, forms patterns in the substrate. The wet etching process is typically carried out by immersing a cartridge holding multiple wafers in an etchant bath. The bath is typically a quartz pot holding, for example, five gallons of etchant. The etchant is usually a combination of chemicals. When the material being etched is titanium silicide, for example, a standard selective etchant includes ammonium hydroxide, hydrogen peroxide, and water. This combination is non-viscous and is easily mixed.
Etchants containing one or more viscous chemicals, in contrast, require frequent and vigorous mixing to keep the chemicals from separating while in the bath. For example, an etchant of sulfuric acid and hydrogen peroxide can be used for cobalt silicide selective etching. Sulfuric acid is viscous and tends to separate from the hydrogen peroxide. Even the exothermic reaction caused by mixing the two chemicals is insufficient to keep the etchant properly mixed. Accordingly, the etchant must be agitated while the wafers are in the bath.
Various techniques have been developed to stir the etchant mixture while the wafers are immersed in the bath. One such technique is to place a magnetic stirrer at the bottom of the bath. Then, a magnetic wheel beneath the bath is used to move the stirrer. However, this technique does not work well when the etchant has a very high viscosity.
Another technique is to manually stir the etchant mixture with a paddle. This technique, however, is very expensive because a person must stir the bath during the entire etching process, typically lasting 20 minutes. In addition, the person may be subject to repetitive motion injury due to the frequent stirring. The person may also be subject to harm from the chemicals contained in the etchant.
Accordingly, there is a need for a way to keep a high-viscosity etchant from separating while in the bath without using manual labor.